For the design of an antenna that executes transmission and reception in parallel, an important item to be considered is to secure isolation between a transmission antenna device and a reception antenna device, so that a signal transmitted from the transmission antenna is prevented from being input into the reception antenna device. When a dual-polarization antenna is used, which enables a transmission signal and a reception signal to generate different polarizations, for example, polarizations that are orthogonal to one another, the isolation may be quite secured. However, a device for further improving isolation has been provided for the dual-polarization antenna.
FIG. 1 is a circuit block diagram of a dual-polarization antenna equipped with an isolation providing device according to an embodiment of the conventional art, and the configuration illustrated in FIG. 1 is almost identical to the configuration disclosed in U.S. Pat. No. 6,141,539.
Referring to FIG. 1, the conventional dual-polarization antenna equipped with an isolation providing device may include a plurality of dual-polarization antenna parts 10 including a transmission antenna device 11 and a reception antenna device 12 that is physically or electrically orthogonal to the transmission antenna device. That is, FIG. 1 illustrates an example of a (perpendicular) array antenna structure. A transmission signal is provided through a first port (P1), passes through a feeder, and is distributed to each transmission antenna device 11 of the plurality of dual-polarization antenna units 10 for provision, and a signal received through a plurality of reception antenna devices 12 may be coupled and output to the second port (P2).
In this structure, a part of a transmission signal provided to the first port (P1) is distributed to a first coupler 21, and is provided to the second coupler 22 through a conductor 24, and the second coupler 22 couples a signal provided through the conductor 24 with a signal output to the second port (P2). In this instance, the performance of the first and second couplers 21 and 22, and the length of the conductor 24 are designed to enable the signal that is transferred to the second port (P2), through the first coupler 21, the conductor 24, and the second coupler 22 for coupling, to have an identical size and to have a difference of 180 degrees in phase, in comparison with an undesired input signal (hereinafter, referred to as an erroneously input signal) that is transmitted from the plurality of transmission antenna devices 11 and is received by the plurality of reception antennas 12.
When it is described in detail, isolation between dual polarizations is measured before installing the isolation providing device. When isolation between bands is measured, a size of an erroneously input signal and a delay of the signal are measured. Subsequently, a size of coupling of the first coupler 21 and the second coupler 22 in the isolation providing device is determined to have a value similar to a frequency-based average value of an erroneously input signal, measured in a state in which the isolation providing device is not included. In addition, the length of the conductor 24 is designed to enable the delay associated with the first coupler 21, the second coupler 22, and length of the conductor 24, to have a difference of 180 degrees in phase, from a frequency-based average value of a delay of an erroneously input signal, measured in a state in which the isolation providing device is not included.
Accordingly, the erroneously input signal that is transmitted from the first port (P1) through the plurality of transmission antenna devices 11, and is received by the plurality of reception antenna devices 12 and transferred to the second port (P2), and a signal transferred to the second port (P2) through the first coupler 21, the conductor 24, and the second coupler 22, may offset one another, and thereby, may be removed.
As a scale of an antenna becomes small, a size of a reflector of a single antenna or an array antenna also needs to be decreased. In general, when a size of a reflector is insufficient, isolation of a dual-polarization antenna becomes deteriorated. Also, a size of isolation between polarizations is not constant in a frequency range in which an antenna operates.
However, the conventional structure of FIG. 1 is effective only when the size of isolation between polarizations is constant through the entire operating frequency range for a transmission signal. Otherwise, isolation between polarizations may not be improved in the entire operating frequency range.